1. Field of the Invention
Embodiments of the present invention generally relate to the processing of substrates for the manufacture of electronic devices thereon. More particularly, this invention relates to a process and apparatus for heating and cooling a substrate in a vacuum chamber.
2. Description of the Related Art
The manufacture of integrated circuits or flat panel displays generally entails performing numerous processes on a substrate in a vacuum chamber, e.g., physical vapor deposition (PVD), chemical vapor deposition (CVD), etch, etc. Prior to processing a substrate in a vacuum chamber, it is often desirable to heat the substrate. Heating of a substrate prior to processing is generally performed in order to remove residual gases adsorbed on the surface of the substrate, particularly water vapor. In addition to a relatively large quantity of water vapor molecules, which deleteriously affect high vacuum levels, adsorbed gases may also include contaminants undesirable for a particular vacuum process, i.e., the adsorbates present on the substrate during some processes may adversely affect the film formed thereon.
Therefore, prior to processing in a high-vacuum chamber, adsorbates are typically removed from the walls of the chamber via a chamber bake-out and from substrates via a heating or “degassing” process. Otherwise, each substrate brought into the high-vacuum chamber would bring relatively large quantities of moisture into the chamber, making the desired vacuum level for the chamber difficult or impossible to maintain. Further, the large quantities of water vapor brought into a high-vacuum processing chamber as adsorbates on substrates will prematurely load the cryogenic pumps, or “cryo pumps,” that are typically used to achieve high and ultra-high vacuum conditions therein. Premature cryo pump loading results in more chamber down-time, because cryo pump regeneration is time-consuming. Any processing in the chamber is stopped, the cryo pump is isolated from the vacuum chamber, and the frozen gases adsorbed thereon are removed by a purge gas, typically heated nitrogen. Because the time to complete the regeneration process is so long, i.e., on the order of several hours, it is important to perform the cryo pump regeneration as infrequently as possible for a high-vacuum processing chamber to have high throughput.
While a degassing process for incoming substrates is helpful for maintaining higher levels of vacuum in any vacuum chamber, for some processes it is more helpful than others. For example, PVD is performed in an ultra-high vacuum chamber, i.e., absolute pressure ≦10−7 Torr, using a magnetron sputtering process by placing a target above the substrate, providing a gas, such as argon, between the target and the substrate, and exciting the gas with a high-voltage DC signal to create ions that strike the target. As the target is bombarded by ions, target atoms are dislodged and become deposited onto the substrate. The dislodged target atoms generally have substantial kinetic energy and when they impact the substrate the atoms tend to strongly adhere to the substrate. Because it is important to the PVD process to maintain high vacuum levels, degassing is generally performed on substrates prior to any PVD processing.
Degassing of a substrate is typically performed by heating the substrate after it has been placed under vacuum but prior to entry into the processing chamber. For smaller substrates, such as 200 and 300 mm diameter silicon wafers, one or more dedicated vacuum chambers are generally provided on a substrate processing platform in which degassing of the substrate may take place prior to other processing, such as PVD or CVD. After degassing, the substrate is then transferred to the next processing chamber without exposure to air. For large-area substrates, such as glass substrates used for flat panel displays that are 1 m×1 m and larger, utilizing additional processing chambers for degassing is generally undesirable due to the added size, i.e., footprint, and expense of a substrate processing system that would result thereby. The term “large-area substrates,” as used herein, refers to substrates which are about 1 m×1 m and larger. Because the large size and shape of large-area glass substrates makes transfers thereof from one position in the processing system to another relatively difficult and time-consuming, substrate transfers are preferably minimized when processing such large-area substrates. A dedicated degas chamber requires additional substrate transfers compared to a combined load lock/degas chamber.
FIG. 1 is a schematic view of a large-area substrate processing platform, system 100. System 100 includes a vacuum load lock assembly 101, a central transfer chamber 102, a substrate transfer robot 104, and one or more vacuum processing chambers 103. Vacuum load lock assembly 101 and the one or more vacuum processing chambers 103 are generally positioned adjacent and in fluid communication with the transfer chamber 102. Vacuum load lock assembly 101 may contain two or more single slot load locks, load locks 101A-C, which may be stacked vertically due to the size of substrate processed by system 100. Load locks 101A-C and the one or more vacuum processing chambers 103 may be fluidly coupled and decoupled from the central transfer chamber 102 by a slit valve, a gate valve, or other vacuum tight sealing mechanism. The load locks 101A-C act as a transition chamber between atmospheric pressure and the process pressure, which may be several orders of magnitude higher vacuum than that achievable by load locks 101A-C. Transfer chamber 102 is typically maintained at a medium or high vacuum level, e.g., on the order of about 10−6 Torr, and acts as a further transition chamber between the low vacuum present in load locks 101A-C, e.g. 10−3 Torr, and the high or ultra-high vacuum present in vacuum processing chambers 103, e.g., 10−9 Torr.
In operation, each of load locks 101A-C is adapted to accept a substrate, isolate the substrate from atmosphere and from transfer chamber 102, pump down to medium or high vacuum, e.g., 10−3 Torr, and heat the substrate to a degassing temperature, for example about 100° C. Each of load locks 101A-C is further adapted to fluidly couple itself to the transfer chamber 102 after degassing, allowing transfer of the degassed substrate to transfer chamber 102 for subsequent processing in one or more of the vacuum processing chambers 103. After processing in one or more of vacuum processing chambers 103, substrates need to be cooled and removed from system 100. For substrates that are not as large as 1 m×1 m, for example 550 mm×650 mm, a second “exit only” load lock assembly has been used for substrate cooling and venting to atmosphere, allowing adequate cooling of a substrate without substantially affecting throughput of the system. Configuring a large-area substrate processing platform with a second load lock becomes increasingly problematic, however, due to cost and platform footprint constraints. Therefore, it is desirable for load locks 101A-C to be adapted for cooling and venting large-area substrates for removal from system 100 as well as for heating and pumping down large-area substrates for processing in system 100.
Efficient production line processing requires rapid movement of the substrates from one chamber to another within a processing system as well as between vacuum environments and atmospheric environments. Hence, the cooling process cannot take too long, otherwise system throughput will suffer-especially if incoming substrates are degassed in the same chamber in which processed substrates are cooled.
FIG. 2 illustrates a partial schematic side view of a substrate S positioned in a load lock 500 above a heating plate 521, wherein load lock 500 serves as a load lock for a large-area substrate processing system, such as system 100, described above in conjunction with FIG. 1. Heating plate 521 is typically constructed of steel or aluminum, contains one or more resistive heating elements, and is designed to act as an evenly distributed radiant heat source for the substrate degassing process. To minimize heating non-uniformity due to edge effect, heating plate 521 typically extends a distance 560 beyond the periphery of substrate S and is separated by a small gap 561 from substrate S, wherein distance 560 is relatively large compared to small gap 561.
During the heating/degassing process, heating plate 521 is adapted to maintain an elevated and substantially constant temperature, e.g., 200° C., in order to heat the entire substrate to approximately 100° C. via radiant heat transfer. The substrate S is preferably heated concurrently with pump-down of load lock 500 to increase throughput. Because the majority of the degassing process takes place under vacuum, virtually all substrate heating takes place via radiative heat transfer 550 from heating plate 521. The degassing process generally takes place relatively quickly, only lasting between about 40 seconds and 60 seconds, and therefore is not throughput limiting for a large substrate processing system, such as system 100. During substrate cooling, however, throughput is deleteriously affected by radiative heat transfer 550 from the heating plate 521. The substantial thermal inertia associated with a heating plate as large as heating plate 521, combined with its presence in a vacuum chamber, significantly retards the rate at which heating plate 521 can cool after the heating elements of the heating plate have been turned off. Although a cooling gas 510 is typically flowed into load lock 500 during the substrate cooling process and the heating elements of heating plate 521 are turned off, substrate S is typically still absorbing heat from heating plate 521 at the same time that it is being cooled, and this substantially slows the cooling process.
Therefore, there is a need for an apparatus and method for efficiently heating and cooling a substrate in a vacuum chamber.